In order to maximize the information content transmitted over a prescribed spectral bandwidth, polarization multiplexing (also known as “dual-polarization”) is being increasingly used with new transmission formats. The underlying idea is that the spectral density (conveniently measured in units of bits/Hz) can be effectively doubled by employing two orthogonally polarized data-carrying signal components sharing the same optical signal bandwidth. Normally, these two orthogonally polarized components are transmitted with substantially the same intensity, rendering the total resultant light effectively unpolarized as seen from a test-and-measurement instrument having an electronic detection bandwidth significantly lower than the symbol rate of the polarization-multiplexed signal, such as is normally the case with Optical Spectrum Analyzers (OSA).
The Optical Signal to ASE Noise Ratio (OSNR) is a conventional measurable characteristic of the quality of a signal carried by an optical telecommunication link. Under normal and proper operating conditions, the OSNR of an optical communication link is typically high, often in excess of 15 dB or 20 dB, or even greater. The dominant component of the noise in an optical communication link is typically unpolarized Amplified Spontaneous Emission (ASE) noise, which is a spectrally broadband noise source contributed by the optical amplifiers in the link.
A method of measuring in-band OSNR, which is applicable to polarization-multiplexed signals, was proposed by LIU (see U.S. Pat. No. 9,008,508). This method characterizes an optical communication signal at the receiver end based on similar measurements performed both at the transmitter end, i.e. before ASE is introduced on the signal, and at the receiver end, where the OSNR is to be characterized. It compares the signal level filtered at two distinct wavelengths at the transmitter end to the same measured at the receiver end to determine the OSNR. An important limitation is that the proposed method is based on the assumption that the signal spectrum does not deform during transmission.
However, spectral deformations, mostly induced by Non-Linear Effects (NLE), have become more frequent in the case of new deployments using polarization multiplexing, because optimum performance in Bit Error Rate (BER) is obtained by increasing the power propagating within the optical fiber to such an extent that non-linear effects can no longer be neglected. The method proposed by LIU is impacted by signals subjected to NLE-induced spectral deformations. Furthermore, overall system performance in terms of BER is not only affected by the ASE-noise level but also from such non-linear effects (see Vacondio et al, “On nonlinear distorsions of highly dispersive optical coherent systems”, Optics Express, Vol. 20, No. 2, pp. 1022-1032 (2012)). Signal quality therefore cannot be assessed based only on the conventionally measured OSNR because proper performance indicators should also account for NLE-induced distortions.
There is therefore a need for a method and a device to determine quality parameters characterizing polarization-multiplexed signals subject to NLE-induced spectral deformation.